Response of two apple cultivars to potassium thiosulphate as a blossom thinner

Although there are a number of chemicals registered for use as thinning agents in apples in Australia, none are suitable for use in organic apple production. With the move towards the use of simple salts that act as blossom desiccants, rather than hormonal type thinning agents, there is more scope for finding suitable chemicals for organic production. With this in mind, trial work was conducted on two apple cultivars to assess the thinning potential of potassium thiosulphate (KTS). KTS was applied to red 'Delicious' trees at 0.5, 1.0 or 1.5% (v/v) at both 20% and 80% bloom. These treatments were compared with an untreated control, a treatment hand-thinned 4 weeks after full bloom, and the industry standard of 80 mg L-1 ethephon applied at full bloom. All treatments reduced crop load compared with the control. KTS achieved better thinning results than the ethephon standard. Increasing the concentration of KTS did not increase the thinning effect. Mean fruit weight and fruit size were increased by all treatments. KTS was applied to 'Royal Gala' trees at 30% and 80% bloom at rates of 0.5, 1.0 or 1.5% (v/v). An untreated control treatment was also included. All KTS treatments reduced crop load significantly compared with the control, with the 1.5% application rate having the greatest effect. Both fruit weight and fruit size were increased by all KTS treatments.